(a) Field of the Invention
This invention relates to an improved cyclic process for the synthesis of urea. More particularly, it relates to an improvement in or relating to a process for subjecting a urea synthesis effluent, which has been obtained by reacting ammonia and carbon dioxide at a urea synthesis temperature and pressure and contains urea, water, unreacted ammonia and unreacted carbon dioxide, to a stripping step using carbon dioxide at a pressure substantially equal to the urea synthesis pressure.
(b) Description of the Prior Art
As one of conventional methods for separating and recovering unreacted ammonia and unreacted carbon dioxide from a urea synthesis effluent obtained in a urea synthesis step, there is known a method in which the urea synthesis effluent is subjected to a stripping step using one of the raw materials, namely, carbon dioxide, at a pressure substantially equal to the urea synthesis pressure, and a separated gaseous mixture of unreacted ammonia and unreacted carbon dioxide is then condensed and recycled to the urea synthesis step (See, for example, Japanese Patent Publication No. 19964/1963). According to this method, the separation of unreacted ammonia and unreacted carbon dioxide is carried out at a pressure substantially equal to the urea synthesis pressure. Thus, this particular method is advantageous in that the condensation heat given off upon recovering the thus-separated unreacted ammonia and carbon dioxide by condensing the same can be recovered at a relatively high temperature level in the form of steam or the like. However, this method is accompanied by some drawbacks which are described below.
First of all, it has been reported that the above method does not allow to increase the NH.sub.3 /CO.sub.2 molar ratio in a urea synthesis step so as to achieve a high degree of the stripping effect by carbon dioxide. According to the aforementioned Japanese Patent Publication No. 19964/1963, it is said to be necessary to select as the NH.sub.3 /CO.sub.2 molar ratio in the urea synthesis step a value not too much apart from 2.0, namely, a value in the range of from 1.5 to 3.5. Example 1, which appears to be the most preferable embodiment, recites about 2.5 as the NH.sub.3 /CO.sub.2 molar ratio.
However, if a value close to 2 is chosen as the NH.sub.3 /CO.sub.2 molar ratio in a urea synthesis step, the conversion ratio of carbon dioxide to urea declines. For example, where the temperature and H.sub.2 O/CO.sub.2 molar ratio as urea synthesis conditions are respectively 185.degree. C. and 0.3, the conversion ratio of carbon dioxide to urea at equilibrium is about 62.5% if the NH.sub.3 /CO.sub.2 molar ratio is 2.5 whereas it increases to 76.9% if the same molar ratio is 4.0. As a result, a considerably great difference arises in that the former NH.sub.3 /CO.sub.2 molar ratio requires a decomposition and separation of unreacted ammonium carbamate in the amount of 0.78 ton per ton of urea to be produced whereas the latter NH.sub.3 /CO.sub.2 molar ratio needs a decomposition and separation of unreacted ammonium carbamate whose quantity is only one half of the unreacted ammonium carbamate resulting from the use of the former NH.sub.3 /CO.sub.2 molar ratio, in other words, 0.39 ton per ton of urea to be produced. Needless to say, the thermal energy required for the production of urea is not limited to that required for the decomposition and separation of such unreacted ammonium carbamate but includes that needed for the separation of unreacted free ammonia. However, the latter thermal energy is little, compared with the former one. Thus, in a stripping step using carbon dioxide which step requires carrying out the synthesis of urea with such a low NH.sub.3 /CO.sub.2 molar ratio that the thermal energy required for the decomposition of unreacted ammonium carbamate tends to become great inevitably. Although a considerable part of the thermal energy used for the decomposition and separation of such unreacted ammonium carbamate can be recovered as low pressure steam in the condensation step of separated ammonia and carbon dioxide, it results in a situation where highly valuable high pressure steam is consumed in great quantity and, instead, low pressure steam of a low value is produced.
Secondly, when a urea synthesis effluent obtained under such a low NH.sub.3 /CO.sub.2 molar ratio condition is subjected to a stripping step using carbon dioxide, the molar ratio of unreacted NH.sub.3 to unreacted CO.sub.2 remaining in a resultant urea solution is generally very small, more specifically, becomes a value close or substantially equal to 2.
The present inventors found that, where the concentration of unreacted carbon dioxide remaining in a urea solution is so high compared with that of unreacted ammonia also remaining in the same urea solution that the NH.sub.3 /CO.sub.2 molar ratio falls below 2.5, an exposure of the urea solution to a temperature of 190.degree. C. or higher will result in considerable increment in the hydrolysis of urea, which is not advantageous to the process, and the formation of biuret, which are undesirable from the viewpoint of urea quality, compared with a urea solution containing unreacted carbon dioxide in a lower concentration.
In the conventional art, as disclosed in Japanese Patent Laid-Open Publication No. 90118/1979, a method has been adopted to solve the above-described drawbacks in which a urea solution obtained by subjecting a urea synthesis effluent to a stripping step using carbon dioxide is rapidly cooled to 155.degree.-175.degree. C. Although this method seems to be effective in suppressing the hydrolysis of urea and the formation of biuret, it should be used with enlarged apparatus, which is adapted to conduct a stripping step using carbon dioxide therein. Also, equipment is needed to adjust the temperature of carbon dioxide to be used for the stripping to 80.degree.-125.degree. C. Since the temperature of CO.sub.2 gas discharged from a carbon dioxide gas compressor used in a urea synthesis process generally ranges from 140.degree. to 180.degree. C., a carbon dioxide gas cooler operable under high pressure (urea synthesis pressure) is required to follow the above recommendation, causing another disadvantage that carbon dioxide gas of a high temperature is cooled and its heat is taken out of the system.